In my experience, accusations of mental illness are the immediate liberal's response to someone disagreeing with them.
And if this was, say 2010, I would agree that this was overused and problematic. Today? Doesn’t happen nearly as often, and usually gets smacked down by fellow leftists. Meanwhile on the Right they’ve been all too free to dismiss inconvenient people on their side as “mentally ill lone wolves”, though that rhetoric seems to have been fading somewhat in the last couple years.
Unless you’re referring to calling people “narcissist”, “narc”, “n-type”, or even just “N”, which is mostly abuse survivor language referring toxic/abusive people in general, not necessarily ones with Narcissistic Personality Disorder. It’s not specifically a liberal thing (conservative abuse survivors use this language as well) but it shouldn’t be used in political discourse regardless.
Just like how they view all humans to be minerals waiting to be harvested.
Just spitballing here, but this reminds me of the rhetoric that left-wing politicians don’t care about minorities, they just pander to them for donations and votes. Even ignoring the fact that some such politicians themselves are minorities (it varies by country, but the Left tends to look a lot closer to a cross-section of the population than the Right does) that’s more of a lack of imagination on your part than anything resembling reality.